Recirculation of a portion of outlet wastewater from a filtration bed is a known concept. More specifically, it is known to divide the outlet flow from a filtration bed so that some of the emerging wastewater is discharged in the usual manner and some is carried back to pass once again through the filtration bed. One form of such recirculation is described in U.S. Patent No. 4,039,451 wherein some of the outlet flow from a filtration bed is carried back for a second pass through the bed.
In the recirculation process of that reference, and in all known forms of relevant recirculation, the recirculated wastewater passes through the very same path in the filtration bed that it did on its initial pass. As a consequence the total wastewater passing through the filtration bed is a single mixture of wastewater making its first pass plus recirculated wastewater sent back for a repeat pass.
The principal object of the present invention is to improve upon the treatment capability of these known filtration systems which involve recirculation of some of the output wastewater from the filtration bed.
The invention provides an improvement in a filtration system wherein wastewater from a septic tank is directed into perforated primary distribution pipes on an upper surface of a filtration bed of granular material, through which bed some of the wastewater from the primary pipes descends to perforated collection pipes at a low level in the filtration bed. Wastewater emerging from the collection pipes is directed through flow divider means from which some wastewater is discharged and the remaining wastewater is recirculated back through the filtration bed. The improvement provides perforated secondary distribution pipes into which that remaining wastewater is recirculated at a level in the filtration bed below the primary pipes and above the collection pipes. Respective coextensive perforated hood elements are arched over the respective secondary pipes to space them from the granular material above and to each side.
The primary pipes are preferably substantially parallel to one another and the secondary pipes are also preferably substantially parallel to one another and to the primary pipes. The hood elements may be of plastic.
The granular material may comprise a first layer of coarse sand at an upper level of the filtration bed under the primary distribution pipes, a second layer of pea gravel under the coarse sand and surrounding the secondary pipes, a third layer of fine particulate material such as sand under the pea gravel and a fourth layer of clean gravel under the fine sand and surrounding the collection pipes. The secondary pipes and the associated hood elements are within the pea gravel layer in the preferred form of the invention.
The direction of flow of wastewater in the primary distribution pipes may be opposite to the direction of flow in the secondary pipes.